A method is known from document FR-2 493 582 wherein the temperature of the primary fluid is regulated automatically by displacement of the groups of control rods according to the power required from the turbine, the current temperature value and a reference temperature. The axial distribution of thermal power (and neutron flux distribution) is regulated automatically by displacing certain groups, selected in an appropriate manner. Finally, the parameter which is representative of the capacity to increase the power of the reactor is controlled by an operator, by adjusting the concentration of boron in the primary fluid by injection either of pure water, or of a concentrated boron solution.
According to this method, a group must be kept inserted permanently in the low part of the core, in order to be able to assure control of the axial distribution of power. This constraint is impossible to meet when the reactor is operating with high power, since all the groups of control rods are then situated in the high part of the core. In this case, the operators must inhibit the automatic regulation of the temperature and of the axial distribution of power in accordance with the above-described method, and must change it over to another operating mode which permits only automatic regulation of the temperature.
This changeover requires stabilisation of the nuclear reactor: the above-described method for regulation of the core therefore does not make it possible to vary the power of the turbine automatically when the sets are situated in the zone of changeover between the two regulation modes. Operation of the reactor “with frequency regulation” is not possible in this zone, since this functioning consists of modulating the power of the turbine in order to maintain the frequency of the electrical network which is supplied by the reactor at 50 Hertz.